Email messages sent from iPhone5 to an Exchange 5.5 server have an unrecognized character set (dec-mcs), which causes the messages to be converted into worthless text attachments.

The following instructions provide an acceptable work-around.
(Yes, I realize that 5.5 is ancient and that any self-respecting IT manager would upgrade. But there are sometimes factors preventing this)

5 Steps total

Step 1: Using REGEDIT On the Exchange 5.5 server, go to the following:

HKEY_CLASSES_ROOT\MIME\Database\Charset

Step 2: Create a new key called dec-mcs

Step 3: In the new dec-mcs key, create the following values

"Codepage"=dword:000004e4
"InternetEncoding"=dword:00006faf

Step 4: No restart is needed. Messages will now come through properly, although the font will not be perfect